Abstract. Interannual variations in background tropospheric trace gases (such as
carbon monoxide, CO) are largely driven by variations in emissions
(especially wildfires) and transport pathways. Understanding this
variability is essential to quantify the intercontinental contribution to
US air quality. We investigate the interannual variability of long-range
transport of Asian pollutants to the Northeast Pacific via measurements from
the Mt. Bachelor Observatory (MBO: 43.98° N, 121.69° W; 2.7 km a.s.l.)
and GEOS-Chem chemical transport model simulations in
spring 2005 vs. the INTEX-B campaign during spring 2006. Measurements of CO
at MBO were significantly enhanced during spring 2005 relative to the same
time in 2006 (the INTEX-B study period); a decline in monthly mean CO of 41 ppbv
was observed between April 2005 and April 2006. A backtrajectory-based
meteorological index shows that long-range transport of CO from the heavily
industrialized region of East Asia was significantly greater in early spring
2005 than in 2006. In addition, spring 2005 was an anomalously strong
biomass burning season in Southeast Asia. Data presented by Yurganov et al. (2008)
using MOPITT satellite retrievals from this area reveal an average CO
burden anomaly (referenced to March 2000–February 2002 mean values)
between October 2004 through April 2005 of 2.6 Tg CO vs. 0.6 Tg CO for the
same period a year later. The Naval Research Laboratory's global aerosol
transport model, as well as winds from NCEP reanalysis, show that emissions
from these fires were efficiently transported to MBO throughout April 2005.
Asian dust transport, however, was substantially greater in 2006 than 2005,
particularly in May. Monthly mean aerosol light scattering coefficient at
532 nm (σsp) at MBO more than doubled from 2.7 Mm−1 in May
2005 to 6.2 Mm−1 in May 2006. We also evaluate CO interannual
variability throughout the western US via Earth System Research Laboratory
ground site data and throughout the Northern Hemisphere via MOPITT and TES
satellite observations. Both in the Northeast Pacific and on larger scales,
we reveal a significant decrease (from 2–21%) in springtime maximum CO
between 2005 and 2006, evident in all platforms and the GEOS-Chem model. We
attribute this to (a) anomalously strong biomass burning in Southeast Asia
during winter 2004 through spring 2005, and (b) the transport pattern in
March and April 2006 which limited the inflow of Asian pollution to the
lower free troposphere over western North America.